One of the problems which can occur in nuclear reactors is a failure of the fuel cladding during in-core service. Based upon all of the data which is currently available, a mechanism has been postulated which defines clad failures with service.
As a result of fuel burn-up, the fuel pellets swell at the ends to form hourglass configurations such that the ends of the pellets expand through the heat transfer gap between the pellet and the cladding and interact with the cladding. This pellet interaction with the cladding causes stress concentrations. Simultaneous with pellet swelling and pellet clad interaction, fission by-products such as iodine, cesium, rubidium and tellurium as well as trace levels of pellet impurities such as water or hydrocarbons (which ultimately decompose to hydrogen) outgas from the pellet. During the outgasing process, pellet interactions with the cladding increase the density of stress concentrations and biaxial stresses at the clad interfaces. Because of the increased stresses, the oxide film at the pellet/clad interface will sheer and rupture. Fission gas by-products and pellet impurities will migrate from the pellet through the helium or other heat transfer media and be absorbed at the areas of oxide rupture. These fission gas by-products and pellet impurities will accelerate the stress corrosion cracking of the clad and may cause clad failure.
Assuming that these mechanisms explain the failure of the fuel element cladding, two salient factors control the clad in-core reliability. These are the pellet/clad mechanical interaction and the by-product/impurity corrosive attack on cladding. Both of these operative factors must be controlled to eliminate clad failure. Mechanical interaction can be eliminated by introducing lubrication to the inside diameter of the clad or the outside diameter of the pellets or to both surfaces. This will prevent stress buildup. By-product/impurity corrosive attack can be eliminated by rendering the by-product/impurities unreactive. This can be accomplished by introducing getters into the region where stress concentrations are liable to be located. Gettering materials will combine with the gaseous, vaporous, or liquid materials and prevent them from reacting with the cladding.
Conventional gettering systems are normally located at the plenum region between fuel pellets in the active fuel stack, or in-fuel getters (mixed with the fuel). Capsuled getters are quite inefficient since the fusion gas must travel to the region where the capsule is located to be isolated. This could be a problem because the reactive gas can migrate past a stress concentrated area or a defect in the rod I.D. surface and actually enter into a reaction at that site. Thus the getter capsule acts as a fission product pump, pumping corrosion or reactive products past defects which could lead to more failures along the rod then would have been possible if the getter capsule was not placed in the rod. Also, isolated getter capsules (i.e., conventional getters) are ineffective against liquid phase products that fuse with clad protective oxides or thermochemically-mechanically induced solid-solid reactions which may occur at rod-pellet interfaces.
The use of getters located at pellet-pellet interfaces are an improvement over isolated getter capsules. However, there are serious drawbacks to fuel stack getters. Firstly, the use of pellet stack getters (e.g., disks at pellet interfaces) reduces the amount of active fuel in the fuel rod reducing the power of the rod. Additionally, the location of getter disks may change the heat transfer at the pellet interfaces. If a ceramic getter is used the pellet interfaces will not allow the hotter section of the fuel pellet to give off its excess heat which can yield hot spots or spiking of fission products. If a conductive getter disk is used the probability of fusion products or gas to migrate to pellet interfaces (where the interfaces are more susceptible to mechanical interaction with clad) is greatly enhanced. This could lead to swelling at pellet to pellet end-faces and could lead to higher pellet gassing (where some gases can transmutate to corrosive elements.
It should be noted that some considerations have to be made concerning the energitic state of the fission gases/impurities in relation to their ability to react with gettering material. The reaction of the fission gas/impurities with solid gettering particles can be classified as an heterogeneous reaction system where the residence time of the gas at the particle site interface controls the overall effectiveness and yield of the reaction of the gas or other fusion product or systems located at specific locations in a rod with the gettering material. If the velocity of the gas molecule is too rapid in the vicinity of the gettering system of this type, the gas will not chemisorb to the getter surface and sufficient periods will not be available which are essential for gettering efficiency. Introduction of gettering agents into the fuel matrixes suffers from the potential of being either inefficient because of the non-steady-state dynamics that occur in a fissioning fuel pellet or ineffective because of the release of fission products after having been gettered by thermal-radiolytic decomposition. This is caused by the generation of large recoil energies during the fissioning process in the fuel matrix which could cause gettered impurities to be released and thus available to migrite to the fuel-clad interface. Also, if a getter is combined with the fuel, it can decrease the power efficiency of the fuel system. Additionally, the structural changes that occur in the fuel pellets are dynamic and varied. Free surfaces may generate by fuel cracking allowing fission products to spike past in-fuel getters and migrate to pellet-clad interfaces. It should also be noted that the introduction of a getter into the fuel pellet may also further reduce the heat transfer characteristics of the pellet causing the fuel to operate at higher temperatures which may lead to increased fission product release.